There's a PDF of a 2003 Matrix parts catalog here at
http://www.genvibe.com/images/misc/manu ... atalog.pdf but it can be hard to find things and doesn't seem to include the pedal.
Sometimes I'll do a google search for the part, such as 2006 toyota matrix accelerator pedal. That will often turn up links to various toyota parts websites, some ebay listings, so me other random sites and often a number of only marginally related things. Sometimes the ebay listings will provide a toyota part number. On the toyota parts sites, I see what they are calling the part (sometimes it's not what one might think) then on that site start a new search for the same model Matrix and the discovered part name.
and all of that assuming a toyota matrix part will be the same for a Vibe. Usually is, except some body stuff, and some HVAC stuff. There are some minor changes between 03/04 and 05-08, and many between 03-08 and 09-10
I haven't seen a GM/Pontiac parts catalog, but it seems that GM has discontinued support for pretty much every Vibe part, the car being 14 years out of production, so knowing a GM part number isn't very helpful anymore